Use explicit conversion methods for stringified numbers

* As @greggroth pointed out in rspec/rspec-core#1086, the formatter
  does not work as expected when the mathn lib is loaded. This lib adds
  the Rational type which is formatted as "n/n" when stringified rather
  than the float form. By explicitly converting the arithmetic to the
  types we wish to stringify, this display is corrected.
* This contains all commits of PR #1087, squashed
  together for easy back-porting to 2-14-maintenance
  and 2-99-maintenance.